Three-dimensional characteristics of rhesus monkey vestibular nystagmus after velocity steps.

In rhesus monkeys we have investigated 3-dimensional orientations of angular eye velocity vectors during vestibular step responses about an earth-vertical axis. Monkey body position with respect to the rotation axis was systematically changed between upright, 90 degrees ear-down, and supine positions to induce oblique eye movements with horizontal, vertical, and torsional components, respectively. Angular eye velocity axes deviated from stimulus axes due to two types of anisotropies in the vestibulo-ocular reflex, that is, gain anisotropy and velocity-storage anisotropy. Deviations of eye velocity vectors from stimulation axes at peak slow-phase velocity could well be predicted by vectorial summation of gains obtained for the cardinal directions (horizontal, vertical, and torsional). During nystagmus decay, time constants after oblique velocity steps showed a tendency to be similar in participating components. However, this adjustment of time constants was incomplete for horizontal-torsional and horizontal-vertical-down vestibular responses, leading to an orientation change of eye rotation axes during slow-phase velocity decay with a bias toward the horizontal eye movement component.